A combustion control apparatus must be constructed so as to ensure safety by stopping the supply of fuel when fuel ignition is not attained within a predetermined safety period of time after the igniting operation has been started. It must also be constructed so as to ensure safety when a safety timer acting to stop the supply of fuel is disabled.
A combustion control apparatus of this kind will be described with reference to FIGS. 1 and 2. In such an apparatus, pre-purge is effected prior to starting of combustion, in which a combustion air blower is driven to supply air for purging unburned gas remaining within a furnace. Description will be specifically directed to such an apparatus provided with this pre-purging function. Description will also be specifically directed to such an apparatus in which its operation sequence includes the step of post-discharge.
Referring to FIGS. 1 and 2, when the temperature of an object to be heated drops, a signal indicative of such a temperature drop is applied through an input terminal 1 to a control circuit 2. This signal constitutes an operation starting signal. In response to the application of this signal, a blower drive signal is delivered through a terminal 3, and a blower starts its operation to start pre-purge. At the same time, in response to the operation starting signal, a timer circuit 4 starts its timing operation by dividing the frequency of clock pulses applied through a terminal 5. Upon lapse of a predetermined period of time (which will be referred to hereinafter as a pre-purge period), a pre-purge termination signal is delivered from the timer circuit 4, and in response to this signal, an igniter drive signal and a fuel valve drive signal are delivered through terminals 6 and 7 to drive an igniter and a fuel valve respectively, thereby commencing the igniting operation. The igniter effects electric discharge. When ignition is attained within a predetermined period of time (which will be referred to hereinafter as a safety period), a flame signal is applied to the control circuit through a terminal 8 to continue the operation, and the electric discharge is still continued for an additional predetermined period of time (which will be referred to hereinafter as a post-discharge period). Thereafter, steady combustion is maintained.
If ignition is not sensed within the safety period, a judgement is made so that there happens a trouble, and an ignition failure signal is generated in response to a safety period termination signal produced from the timer circuit 4, and a safety unit drive signal is delivered through a terminal 9 so that a safety unit is operated to stop the blower, fuel valve and igniter, as shown by broken lines in FIG. 2, for ensuring the safety.
Thus, when ignition fails to take place, the safety period termination signal appearing from the timer circuit 4 acts to cut off the fuel valve. Therefore, if a trouble occurs in the timer timing the safety period, the safety period termination signal would not be delivered, and fuel would be continuously discharged in the event of ignition failure, resulting in a serious danger.
In the prior art practice, two timers each timing this safety period have been provided to deal with such a dangerous situation. Therefore, a timer for timing the pre-purge period, a timer for timing the post-discharge period and two timers for timing the safety period, that is, a total of four timers have been necessarily required heretofore.